A laterally variable optical filter (LVF) may include a transmission optical filter having a transmission wavelength varying in a transverse direction across a surface of the filter. A compact optical spectrometer may be constructed by coupling a photodetector array to an LVF. When the LVF surface is illuminated with light reflected by the sample, light portions at individual wavelengths are selectively transmitted through the LVF and detected by individual pixels of the photodetector array. As a result, a reflection spectrum of the sample may be obtained. A miniature light source may be provided in an LVF-based spectrometer for illuminating a sample, enabling quick, on-the-spot reflection spectral measurements.
Due to their miniature size and small weight, LVF-based spectrometers may be constructed in handheld configuration, making them suitable for use in a variety of test and measurement applications. It is sufficient to bring such a handheld LVF-based spectrometer in close proximity with the sample, and to press a thumb-operated button on the back of the spectrometer to quickly (within seconds, or even less than a second) obtain a reflection spectrum. Other types of spectra, e.g. fluorescence, transmission, etc., may also be obtained with a suitable configuration of the light source.
A direct contact between the handheld spectrometer and the sample may be undesirable. For instance, chemically aggressive samples, extremely hot or cold samples, bio-hazardous samples, etc., may damage the compact spectrometer or endanger the user. In other applications, such as food processing, a direct physical contact may be undesirable for hygienic reasons. For applications such as these, a remote spectra collection may be preferable.
To obtain a spectrum from a safe distance, the illuminating light may be formed into a parallel or converging beam of light by a lens-based or mirror-based collimator, so as to concentrate the illuminating light on the sample. A mirror or lens, or a lens system, may be used to remotely collect light reflected by the sample. Alternatively, a pair of optical fibers may be used to bring the illuminating light to the sample, and to guide the reflected light back into a spectrometer for a spectral measurement.
Free-space or fiber-guided illumination delivery and light collection systems have some drawbacks. For repeatability purposes, a free-space collection system needs to be placed at a pre-defined distance from the sample, which may not be convenient, or may even be unfeasible when holding the spectrometer by hand. Light collection systems based on optical fibers typically require precision optical alignment for coupling light into the illuminating fiber, and may require dedicated optics, so-called “optical head”, for coupling the reflected light into a core of the light collection fiber. Transmission of a fiber bundle may depend on its bend radius, requiring re-calibration if the fiber bundle is re-shaped or bent at a different radius. Furthermore, most optical fibers are fragile and expensive, and may require replacement upon damage of sensitive fiber tips caused by chemically aggressive, hot, cold, or abrasive samples.